Most geological structures relevant to oil and gas production retain between 70% to 90% of their original hydrocarbon stores after primary production driven by natural reservoir pressure release is complete. Hydraulic fracturing is often used to increase reservoir contact and increase production rates. During the fracturing process, proppants are typically added to a fracturing fluid pumped into the geological structure in order to keep the fractures from closing in upon themselves when pressure is released. Another technique commonly used in secondary production is displacement flooding, of which water-flooding is the most common. In flooding techniques, a displacing fluid is introduced from an injection well, and oil and/or gas are extracted from a nearby production well. The displacing fluid frees oil or gas not released during primary production and pushes the oil or gas toward the production well. Displacing fluids include, for example, air, carbon dioxide, foams, surfactants, and water. Hydraulic fracturing is often applied to injection and production wells in conjunction with displacement flooding operations.
In spite of the undisputed utility of hydraulic fracturing and water-flooding in petroleum production processes, few methods exist for monitoring the extent and quality of the fracturing and flooding processes. Fractures can be monitored and approximately mapped three-dimensionally during the fracturing process by a ‘micro-seismic’ technique. The micro-seismic technique detects sonic signatures from rocks cracking during the fracturing process. The setup of this technique is prohibitively expensive, and data that is generated tends to be relatively inaccurate due to high background noise. Further, the process can only be performed during the fracturing process and cannot be repeated thereafter. Water-flood operations can be monitored with low resolution through four-dimensional seismic surveys. As the density difference between water and petroleum is small, the flood front is not abruptly distinguishable, and the imaging resolution tends to be on the order of tens of meters. Unlike the micro-seismic technique for monitoring fracturing, flooding operations can be measured periodically to monitor flooding progression.
Neither of the above techniques have the capability to accurately determine the size, structure and location of injected materials such as, for example, injected proppants and water-flood. Improved knowledge concerning the location of injected proppants and water-flood in fractures and natural geological pores would aid production engineers in tailoring production conditions to meet local geological settings. Further, knowledge about the location of injected proppants and fractures would significantly improve safety in production processes by identifying potentially catastrophic events before their occurrence. For example, vertical fractures can rupture the strata sealing geological structures and potentially intersect fresh water aquifers. Detecting a vertical fracture situation would allow production wells to be sealed, thereby preventing petroleum loss and aquifer damage.
In view of the foregoing, improved methods for imaging geological structures are needed. Such methods would include the capability to obtain high-resolution images of fractures and injected materials, as well as the ability for numerous measurement repetitions to be made. Utilizing such imaging methods solely or in combination with existing geological assays, production engineers could take measures to extract residual petroleum from a geological structure if it is determined that un-extracted hydrocarbons remain after production stimulated by fracturing and flooding operations or a combination thereof is complete.